I'm assuming they didn't just go out in the wilderness with a net hunting for negroes. My (current) understanding that at, at least in the earliest years of the slave trade, they actually purchased from or traded with the African tribes themselves for slaved. So what was the actual methods used to get slaves back then?
While you wait for experts on the topic, I'll link and repost (with some rephrasings) a prior answer I wrote about this:
Yes, West African kingdoms and polities (not tribes) were indeed the main exporters of slaves throughout the Atlantic slave trade.
There already were slaves in West Africa even before European influence, but it was pretty different from, say, slavery in 18th-century Haiti; for example, many slaves in Africa actually did very little, and were there to show the wealth and power of the owner. Much slave labor was "occasional work performed as a tax or tribute," and slaves were more likely to be eventually freed. Traditional African slavery had aspects that we would now think brutal, but it was generally less brutal than chattel slavery in the Americas. The slaves could be criminals, voluntary slaves, but especially, war captives.
When the Europeans arrived, the slave "production" increased. Slaves included "scofflaws, tax cheats, political exiles, [and] unwanted immigrants," and sometimes kings and leaders hiked criminal penalties to have more criminals and thus have more slaves to sell. But in general, most slaves came from war: war captives, from raids or battles. Sometimes an important person in the enemy polity was abducted and a ransom of, of course, slaves was demanded.
The Africans got weapons, manufactured goods, and other valuable things in exchange. African demand was as important as European demand; for example, Asante traders brought slaves to a Dutch fort in 1732 and demanded that the Dutch buy the slaves and sell their flintlock guns in exchange (flintlocks were superior to matchlock guns). The Dutch governor felt threatened, and had to order all flintlocks to be sent to the fort at once. The rise of Dutch export in slaves in the 1720s may be directly linked to Asante selling slaves and buying arms to have a competitive edge against its neighbors. This was not limited to Asante; for example, the King of Dahomey also obtained much of his wealth from the slave trade.